Page 371 - Power Electronics Handbook

Power supplies 36 I

supply , 1 i sw, b Load
A.C.
-
mains
ihroratm 1

I

Battery Battery Inverter Filter j SW, J I
charger
I

provide emergency supplies to vital equipment such as computers and
life-support systems. The changeover from the mains to standby supply must
occur almost instantaneously, hence the term ‘unintermptable power
supplies’ is used when describing this type of equipment. They are also
called standby power supplies, since they stand by whilst the mains is
working. These power supplies usually consist of inverters operating from a
d.c. storage source, such as a battery, since such a supply can be brought into
service quickly, and it is switched off once a motor generator is able to run
up to speed to supply the power. Furthermore, not all emergency loads
require unintermptable supplies: emergency lighting, for example, can
usually operate effectively with a break of about 200ms.
Figure 14.8 shows the schematic of a typical uninterruptable power
supply. The load is normally supplied direct fram the main ax. supply,
switch SW1 being closed and SW2 being open. During this period the
battery is trickle charged from the mains supply. The inverter is operated
at a fixed frequency and is therefore optimised for a sine wave output, by
using techniques such as pulse-width modulation or waveform synthesis,
the output waveform being further enhanced by the filter. The inverter is
also synchronised to that of the mains so that when the main supply fails,
switch SW2 closing to apply the inverter supply to the load, there is no
waveform distortion.

14.2.2 Variable-speed constant-frequency supply
A variable-speed constant-frequency (VSCF) supply provides an accurate
fixed-frequency output voltage from a variable-frequency source. Such a
system is used in aircraft where the main a.c. power is generated by
an alternator driven from the aircraft’s engines, and since these operate at
variable speeds the output from the alternator has a variable frequency.
Traditionally, variable-slip couplings have been used between the engine
and the alternator to keep its speed constant, but such mechanical systems
need maintenance and have limited life. A better solution, shown in Figure